1. Field of the Invention
The present invention relates to the protection of motor windings and, more particularly to the protection of start windings of permanent split capacitor motors due to over-current conditions.
2. Description of the Prior Art
In the past, various types of combination starter-protector devices have been utilized in circuit relation with a winding circuit of a motor. At least some of these past combination starter/protector devices utilized a positive temperature coefficient (PTC) element which was operable generally to at least impede current flow to a start or auxiliary winding of the motor circuit generally as the motor was energized to its preselected running or synchronous speed, and an overload or protector was thermally coupled with the PTC element and subjected to the current in at least a run or main winding of the motor. Of course, the overload was operable to interrupt the current flow to the motor in the event of the occurrence of a winding circuit overload condition which may have a deleterious effect on components of the motor, such as for instance, burning out of the winding circuit.
In existing single phase induction motors, such as capacitor start induction motors and split phase type motors, a PTC element has come to be used as a switching element for energizing the start winding of the motor and disassociating the start or auxiliary winding from the run or main winding of the motor when the motor attains its running speed. Upon initial energization of the motor, the PTC element is at a low temperature, and sufficient current passes through the PTC element to energize the start winding to thereby assist in the energization of the motor. A PTC element is self-heating upon energization wherein the resistance of the PTC element increases generally as a function of its increasing temperature. Therefore, at elevated temperatures of the PTC element caused by the passing current, the PTC element acted to limit passage of current applied therethrough to the start winding to an acceptable minimum value which, in effect, electrically disassociated the start winding from circuit relation with the run or main windings of the motor generally upon the motor attaining its preselected running speed.
One of the disadvantages or undesirable features of employing a PTC element as a starting resistor is the time required, after the motor was de-energized or turned off, for the PTC element to cool sufficiently in order to restart the motor. In some instances, at least one or two minutes may have been necessary to cool the PTC element to a temperature value at which it may pass sufficient current to effect motor restarting.
Permanent split capacitor motors (PSC's) with or without a start capacitor, which are used in most single phase room and unitary compressors, do not switch out the winding during normal operation. Such motors offer a desired compromise between starting torque, running efficiency, and cost requirements for these horsepower sizes. The use of PTC elements were not desirable in PSC's because switching out of the auxiliary winding was not desired. Thus, protector devices for PSC's to cut out the auxiliary winding and/or the main winding from the circuit remained thermally responsive contactors in thermal relation with a resistance heating wire.
The disadvantages of a thermal contactor thermal wire protector is its inherent inability to quickly disassociate the windings from the circuit, and its long period of time before reset. The windings could be damaged to a point that operation would not or should not occur.